Sublimation of 2,6-and 2,7-dimethyl-naphthalene mixture

ABSTRACT

1. METHOD OF SUBLIMATING A SOLID NONEUTECTIC MIXTURE OF DIMETHYLNAPHTHALENES CONSISTING ESSENTIALLY OF 2,6-DIMETHYLNAPHTHALENE AND 2,7-DIMETHYLNAPHTHALENE COMPRISING: (A) ELEVATING THE REMPERATURE OF THE SOLID, SUBLIMAND MIXTURE TO BETWEEN AMBIENT TEMPERATURE AND THE MELTING POINT OF AN EUTECTIC MIXTUREOF 2,6-DIMETHYLNAPHTHALENE AND 2,7-DIMETHYLNAPHTHALENE; MAINTAINING THE ELEVATED TEMPERATUREUNTIL THE COMPOSITION OF THE INITIAL, SOLID MIXTURE HAS CHANGED; AND (B) REMOVING THE VAPORS ARISING FROM THE SUBLIMAND AND COLLECTING THE SUBLIMATE.

United States Patent 3,839,479 SUBLIMATION OF 2,6- AND 2,7-DIMETHYL- NAPHTHALENE MIXTURE John A. Hedge, Wilmington, Del., assignor to Sun Research and Development C0,, Philadelphia, Pa. No Drawing. Filed Sept. 18, 1972, Ser. No. 289,900 Int. Cl. C07c 7/02 U.S. Cl. 260-674 N 7 Claims ABSTRACT OF THE DISCLOSURE A solid noneutectic dimethylnaphthalene mixture containing 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene along with smaller amounts of other hydrocarbons can be sublimated whereby the remaining solid consists essentially of 2,6-dimethylnaphthalene and 2,7-di methylnaphthalene.

BACKGROUND OF THE INVENTION This invention relates generally to a method for fractionating difficult-to-separate C bicyclic aromatic hydrocarbons and in particular, isomers of dimethylnaphthalenes. More specifically, it relates to a method for fractionating 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene by sublimation.

Dimethylnaphthalenes are oxidized to naphthalenecarboxylic acids which are used in the production of dyes, polyesters, pigments, etc. A more detailed discussion of the utility of dimethylnaphthalenes appears in Naphthalenecarboxylic Acids by K. A. Scott in Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Edition, Volume 13.

For convenience, dimethylnaphthalene or dimethylnaphthalenes herein will be referred to as DMN with specific DMN isomers being indicated by reference to the location of the methyl groups. For example, 2,6-dimethylnaphthalene will be referred to as 2,6-DMN.

DMN are found in coal tar, lignite tar, crude oil, the drip-oil fraction produced during the pyrolysis of hydrocarbons to make olefins, in heavy petroleum reformate and in petroleum gas oil produced by catalytic cracking. In these hydrocarbon mixtures, DMN are usually present in rather dilute concentration. For example, one analysis shows DMN making up about 4 percent by weight of a gas oil. However, by known processes such as distillation, crystallization, extraction and complexation, a mixture consisting essentially of 2,6-DMN and 2,7-DMN can be recovered in concentrated form from the previously mentioned sources. A recent example of a complexation is described in U.S. Pat. 3,670,039, issued June 13, 1972, to R. I. Davis.

Since both 2,6-DMN and 2,7-DMN have a boiling point of 263 0., further distillation of such a mixture is impossible.

In addition, by the aforementioned methods, the resulting concentrated DMN can be a binary eutectic of DMN; i.e., 2,6- and 2,7-DMN. The weight or mole concentration of such a eutectic is 41.5% 2,6-DMN and 58.5% 2,7-DMN. Its eutectic freezing point is about 71 C., whereas the melting point of 2,-6-DMN is 112 C., of 2,7-DMN is 97 C. However, sublimation of such an eutectic mixture does not result in a composiiton change of the treated solid. The isomers of such an eutectic can be separated by the complexation method described in U.S. Pat. 3,665,045, issued May 23, 1972, to R. I. Davis et a1.

Yet in oxidizing the two dimethylnaphthalenes to naphthalene carboxylic acids, it is preferable that each isomer be oxidized by itself since each one requires somewhat dilferent reaction conditions for optimum oxidation. Also, a single acid can be wanted. Thus, there is ice a need for a method for fractionating a 2,6-DMN-2,7- DMN mixture.

SUMMARY OF THE INVENTION This invention relates to a method for the fractionation of a noneutectic DMN mixture consisting essentially of 2,6-DMN and 2,7-DMN. The temperature of a normally solid mixture of 2,6-DMN and 2,7-DMN is raised to a temperature below the melting point of a 2,6- DMN-2,7-DMN eutectic mixture and maintained at the elevated temperature until a desired concentration of the solid isomer is obtained. At the elevated temperature, undesired hydrocarbons sublimate as well as some of the desired isomer.

DESCRIPTION OF THE INVENTION The noneutectic DMN mixture treated via this invention consists essentially of 2,6-DMN and 2,7-DMN. Other compounds of the 2,6-DMN-2,7-DMN mixture can be other DMN, ethylnaphthalenes, moncyclic aromatics, tetrahydronaphthalenes, indane and other similar hydrocarbons. These other compounds are present in small quantities. At 20 C., the DMN mixture is solid. Surprisingly, for example, it is most, if not all, of the other compounds that first vaporize when a predominantly 2,6- DMN mixture is treated via this invention.

Fractionation of the solid DMN can occur at normal ambient temperature; however, the slow rate would make it economically unattractive. Therefore, an elevated temperature is preferred. However, if the temperature were to exceed the eutectic freezing point, the composition of the vapor could change thereby adversely changing the composition of the residual. Thus, while an operative temperature range is between ambient temperature and the eutectic freezing point of a 2,6- and 2,7-DMN mixture, preferably, the lower temperature limit would be 2030 higher than ambient temperature, whereas the upper temperature limit would be 510 lower than the eutectic point.

The elevated temperature is maintained until the com position of either the solid or vapor obtains the desired composition. With a noneutectic 2,6-2,7-DMN mixture,

'for example, normally the elevated temperature would be maintained until all the other compounds vaporize and either mostly 2,6- or 2,7-DMN remain. However, the elevated temperature could be maintained until not only are all the other compounds vaporized, but, for example, sufficient 2,7-DMN, in conjunction with some 2,6-DMN, has vaporized such that the remaining solid has the desired concentration of 2,6-DMN; for example, 98%- 99%.

Also, the fractionation of the solid DMN can be performed not only at atmospheric pressure, but also at a positive pressure or under reduced pressure. Economics would dictate what pressure would be used.

During fractionation of the noneutectic mixture an inert gas can be passed over or through the mixture. The temperature of the inert gas, such as air, nitrogen, carbon dioxide and others can be at the proper elevated temperature. Alternatively, the temperature of the solid mixture can be, via radiation, at the proper elevated temperature with the passing inert gas at ambient temperature. Another alternative would be to use, for example, both radiation and a heated inert gas.

The solid, noneutectic mixture during the fractionation could be one large solid piece like the cake, in a slab form, from a filter press. Or, for example, the same slab could be broken or crushed into numerous smaller pieces. The latter would be more operative.

The vapors from the solid noneutectic mixture would be normally cooled whereby the vapors would condense and the vaporized hydrocarbon could be recovered. If

an inert gas was used, then the mixture of inert gas and the vapor could be subsequently cooled whereby the vaporized hydrocarbon would condense to a solid facilitating recovery. In either of the two aforementioned systems, the recovered hydrocarbon could be processed further via, for example, crystallization, or recycled for another fractionation via this invention.

Following are examples of present invention and, in addition, one nonoperative demonstration.

EXAMPLES In Run I, five parts of a solid DMN mixture were placed in a typical vertical sublimator with a liquid cooled condenser. The composition of the solid mixture, also referred to as Feed, had the composition shown in the accompanying Table. The vertical sublimator is described in the text book, Distillation, 2nd Edition, E. S. Perry A. Weissberger, Interscience Publishers, Chapter VIII. The sublimator was operated at a temperature of 49 C. and under a reduced pressure of 0.1 mm. of Hg. The vapors were collected and analyzed. As shown in accompanying Table, the first vapor fraction, some 0.52 parts contained 31.5 weight percent of other compounds. These other compounds are hydrocarbons such as other DMN, moncyclic aromatics, tetrahydronaphthalenes, indane and other similar hydrocarbons. These other compounds were not removed by the preceding purification steps. The amount of other compounds contained in the second vapor fraction (9.5%) was substantially less than that contained in the first vapor fraction (31.5%). In the third vapor fraction, no other compounds were found by gas liquid chromatography. The residue, that is the material remaining after the sublimation, contained mostly 2,6-DMN and only minor amounts of 2,7-DMN. The melting point of the residue was 109.5-110" C.

In Run II, five parts of the same DMN mixtu re as used in Run I was placed in a similar vertical sublimator. In this Run H, the sublimator was operated at a higher temperature, 60 C., compared to 49 C. in Run I. The reduced pressure was 0.1 mm. of Hg.

As can be seen from the Table, because of the higher temperature of Run II, the second vapor fraction contained no other compounds. The residual analyzed 97% 2,6-DMN, 3% 2,7-DMN and no other compounds. The melting point of this residue was 106l08 C.

In Run III, five parts of a mixture of DMN were placed in a sublimator. This mixture contained 90% 2,7- DMN and 10% 2,6-DMN whereas in Runs I and II, the mixtures were predominantly 2,6-DMN. In Run III, the operating temperature was 50 C. and the operating pressure was 0.075 mm. of Hg. As can be seen from the Table, the first vapor fraction contained 31.4% of 2,6- DMN compared to 10% in the feed and the fourth vapor fraction contained 13.6% of 2,6-DMN. As a result of the sublimation, the residue contained only 1.7% of 2,6-DMN compared to 10% in the feed. Thus Run III demonstrates that the concentration of 2,7-DMN in a DMN mixture containing predominantly 2,7-DMN can be increased to above 98%.

In Run IV, an eutectic mixture of 2,6-DMN-2,7-DMN was placed in a vertical sublimator similar to that used in Runs I and II. This eutectic mixture contained no other compounds. As can be seen from the data in the accompanying Table, the vapor fraction has the same composition as the Feed. Thus, sublimation of an eutectic mixture of 2,6-2,7-DMN causes no fractionation.

TABLE.ANALYSIS OF FEED, VAPOR, AND RESIDUE FRACTIONS VIA SUBLIMATION OF DMN MIXTURE Composition, weight percent Amt,

parts 2,6-DMN 2,7-DMN Other Run I4) C. and 0.1mm.:

Feed 5.00 89. 9 6.1 4.0 1st vapor fraction 0. 52 47. 5 21.0 31. 5 2d vapor fraction 0.42 81. 5 9.0 0. 5 3d vapor fraction 0. 41 91.0 9.0 None Residue 3.65 98.0 2.0 Run IIGO O. and 0.1 mm;

eed 5.00 89. 9 6.1 4. 0 1st vapor fraction 0.60 44.0 21.7 34. 3 2d vapor fraction 0.83 94. 0 G. 0 None Residue 3. 57 97. 0 3. 0 Run III-50 C. and 0.075

Feed 5. 00 10.0 1st vapor fraction 0.35 31.4 2d vapor fraction 0. 56 23. 2 3d vapor fraction. 0.75 20.0 4th vapor fraction. 0. 14 13.6 Residue 2. 90 1. 7 Run IV--49 c. and 0.1 mm.:

Feed 41. 5 Vapor fraction 41. 5

The invention claimed is:

1. Method of sublimating a solid noneutectic mixture of dimethylnaphthalenes consisting essentially of 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene comprising:

(a) elevating the temperature of the solid, sublimand mixture to between ambient temperature and the melting point of an eutectic mixture of 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene; maintaining the elevated temperature until the composition of the initial, solid mixture ha changed; and

(b) removing the vapors arising from the sublimand and collecting the sublimate.

2. Method according to Claim 1 wherein the sublimand contains a major amount of 2,6-dimethylnaphthalene and after sublimation the remaining solid contains at least about 98 weight percent of 2,6-dimethylnaphthalene.

3. Method according to Claim 1 wherein the sublimand contains a major amount of 2,7-dimethylnaphthalene and after sublimation the remaining solid contains at least about 98 weight percent of 2,7-dimethylnaphthalene.

4. Method according to Claim 1 wherein at least during the elevated temperature the solid, sublimand mixture is under reduced pressure.

5. Method according to Claim 1 wherein inert gas is used to change the temperature of the solid, sublimand mixture.

6. Method according to Claim 5 wherein the inert gas is collected and the sublimate contained therein is recovered.

7. Method according to Claim 6 wherein the inert gas is selected from the group consisting of air, nitrogen and carbon dioxide.

References Cited UNITED STATES PATENTS 3,202,726 8/1965 Malrnberg et a1. 260674 1,685,624 9/ 1928 Andrews 260674 1,836,211 12/1931 Weiland et al. 260674 3,173,960 3/1965 Robinson 260674 3,485,885 12/ 1969 Peterkin et al 260674 3,590,091 6/1971 Skarada et al. 260674 DELBERT E. GANTZ, Primary Examiner C. E. SPRESSER, JR., Assistant Examiner 

1. METHOD OF SUBLIMATING A SOLID NONEUTECTIC MIXTURE OF DIMETHYLNAPHTHALENES CONSISTING ESSENTIALLY OF 2,6-DIMETHYLNAPHTHALENE AND 2,7-DIMETHYLNAPHTHALENE COMPRISING: (A) ELEVATING THE REMPERATURE OF THE SOLID, SUBLIMAND MIXTURE TO BETWEEN AMBIENT TEMPERATURE AND THE MELTING POINT OF AN EUTECTIC MIXTUREOF 2,6-DIMETHYLNAPHTHALENE AND 2,7-DIMETHYLNAPHTHALENE; MAINTAINING THE ELEVATED TEMPERATUREUNTIL THE COMPOSITION OF THE INITIAL, SOLID MIXTURE HAS CHANGED; AND (B) REMOVING THE VAPORS ARISING FROM THE SUBLIMAND AND COLLECTING THE SUBLIMATE. 